The present invention relates to thermally driven pumps. More specifically, one embodiment of the present invention relates to the use of a thermoelectric material to create a thermally driven, bi-directional pump, such as a micro pump, with no moving parts using the thermal transpiration effect (a Knudsen pump). The thermally driven Knudsen pump of the present invention achieves high flow rates in both directions, is easy to fabricate, generates a continuous pneumatic pressure, and exhibits an increased pump efficiency over prior art Knudsen pumps. A second embodiment of the present invention relates to Knudsen pumps utilized on micro fluidics platforms, particularly Knudsen pumps integrated and configured within the substrate of lab-on-chip devices and further configured to pump liquids through channels also configured within the substrate of micro fluidic devices in response to pressure generated within the channels of the Knudsen pumps resulting from thermal transpiration.
Knudsen pumps on the macro scale operate at pressures lower than atmospheric pressures. Because micro scale Knudsen pumps operate at atmospheric pressure (pumps with channels having a hydraulic diameter of 100 nm or less), Knudsen pumps have recently been utilized in micro scale applications. Macro-scale Knudsen pumps can operate at atmospheric pressure so long as they contain channels with a hydraulic diameter of 100 nm or less.
Microtechnology is technology whose smallest feature is less than one millimeter in size. Micromechanical systems, sensors, actuators, and pumps, for example, are increasingly being utilized in microtechnology applications. For example, Micro-Electro-Mechanical Systems (“MEMS”) refers to the integration of mechanical elements, sensors, actuators, pumps, and electronics on a common silicon substrate through micro fabrication technology. Fluid flow in microtechnology and MEMS applications is typically accomplished by external pressure sources, external mechanical pumps, or internal micro pumps.
For example, micro gas pumps find a variety of uses in microtechnology and MEMS ranging from gas manipulation, forced convective cooling, micro plasma, and gas chromatography to microfluidic applications like Lab-On-Chip, protein immunoassays and active micro mixers. In many applications such as filtration, by-pass medical devices, and micro total analysis systems, it is desired to have a pump that is bi-directional. Electrostatically actuated diaphragm pumps are commonly used micro pumps, but reliability can be a concern due to moving parts that are more prone to wear and tear, particularly in the micro scale.
Regarding micro fluidics platforms, micro fluidic devices have a tremendous potential in biomedical and chemical applications. They provide miniaturized platforms for fluidic handling, separation, mixing, dilution, etc. Some of the advantages of these devices are low cost, low sample requirement, fast response time, parallel processing and repeatability. One key component of all micro fluidic devices is a means to transport the liquids. This is typically performed with a micro fluidic pump.
Because of the high demand, many types of micro fluidic pumps have been developed including diaphragm pumps, electro hydrodynamic pumps, and electro osmotic pumps. However, most of these pumps suffer from some drawback, such as requiring a high voltage, working only within a certain pH value, requiring a non-sieve that results in filtration of the liquid, large size, and compatibility issues with the underlying micro fluidic device. External pneumatic pumps are commonly used in conjunction with valves, either integrated micro valves, or a plurality of external valves due to the difficulties of fabricating high quality micro valves. There is a need for a micro fluidics device that overcomes these challenges. There is also a need for a micro fluidics device that is configured so that it may be readily integrated with a micro fluidic channel, uses a low voltage, can pump any liquid through pneumatic actuation, and doesn't require any moving parts.
In the last few years, the Knudsen pump has been demonstrated at the micro level for pumping gases. Thermo molecular pumps, such as the Knudsen pump, inherently have a high reliability because they have no moving parts. The Knudsen pump also features a simplified fabrication process (no moving parts), continuous flow, and low operating voltages. The Knudsen pump operates on the principle of thermal transpiration. In 1910, Knudsen demonstrated the possibility of using thermal transpiration for the purpose of gas pumping. The phenomenon of thermal transpiration induces a pressure difference in a narrow channel, whose dimension allows the gas flow in free molecular or transition regimes to become rarefied, when a thermal gradient is established along the channel. A parameter called Knudsen number (Kn) is defined as the ratio of the mean free path of the gas molecules to the hydraulic diameter of the channel and its range for the above mentioned gas flow regimes is 0.1<Kn<10 and Kn>10 respectively. Optimally, the Knudsen number should be greater than 1 for the Knudsen pump to operate efficiently. For operation of a Knudsen pump at atmospheric pressure, channels that have a hydraulic diameter of less than 100 nanometers should be used. When larger channels are used, the pump is operated at pressures lower than atmospheric pressure. Generally if two containers are filled with the same gas separated by a narrow channel and kept at different temperatures, they settle at different pressures.
As Knudsen illustrated, when two large volumes are interconnected by a channel of very small cross-section, of radius smaller than the mean free path length of the gas molecules present, and when the ends of the channel are at different temperatures, then a pressure difference is established between the two large volumes. In the small-sized channel, molecules move under molecular conditions, and as a result the pressures differ at the two ends of the channel because of the temperature difference. Under molecular conditions, when thermal equilibrium is reached, then the pressures at the two ends of the channel are such that the ratio is equal to the square root of the ratio of the corresponding temperatures.
When the molecules reach the large volume adjacent to the hot end of the channel, their travel no longer occurs under molecular conditions, but occurs under viscous medium conditions. As a result, at the hot end of the channel, the molecules escape from the channel and penetrate into the adjacent large volume. This produces a pumping effect with a compression ratio that can be as great as the square root of the temperature ratio.
The flux of gas molecules going through the channel is represent by pressure as ‘P’, mass of the gas molecules as ‘M’, Boltzmann's constant absolute temperature as ‘Γ’ and subscripts ‘h’ and ‘c’ denote hot and cold chamber terms respectively. Once the equilibrium is reached, the ratio of the pressures of the hot and cold chambers is equal to the ratio of the square root of their absolute temperatures. This is represented by the following formula:
            P      h              P      c        =                    T        h                    T        c            
In the free molecular flow regime the intermolecular collisions are negligible compared to the interaction of molecules with the walls of the channel. The average speed of molecules arriving from the hotter region is larger than those from the colder region. The mass fluxes from the two regions balance at equilibrium. With an increase of pressure in the chamber of higher temperature, an inverse flow is driven by the pressure difference of the two reservoirs. The two flows are counterbalanced at some pressure difference.
The mass flow rate through a single channel was derived by Sharipov, and is:
where r is the hydraulic radius, L is the length of the channel subjected to a temperature difference of ΔT, ΔP is the pressure difference across the channel, and Tav and Pav are the average temperature and pressure within the channel. The two flow coefficients, the thermally driven flow coefficient, Mt, and the pressure driven flow coefficient, Mp, are functions of the Knudsen number.
      M    .    =            P      av        ⁢                  m                  2          ⁢                      kT            av                                ⁢                  π        ⁢                                  ⁢                  r          3                    L        ⁢          (                                                  Δ              ⁢                                                          ⁢              T                                      T              av                                ⁢                      M            t                          -                                            Δ              ⁢                                                          ⁢              P                                      P              av                                ⁢                      M            p                              )      
The maximum flow rate at ΔP=0 is a function of the channel length and the temperatures at the ends of the channel. To obtain the maximum flow rate there has to be an optimization between the channel length and the temperature difference obtained along the channel length, as they are interdependent. A longer channel will provide a larger temperature difference, but reduce the flow rate; while with a shorter channel the temperature difference will be reduced, which in turn will drop the flow rate.
In a micro fluidics environment, in applications that require increased pressure drop multiple micro fluidics pumps in series have been used to increase either the temperature gradient or channel length (e.g. U.S. Pat. No. 7,572,110). However, previously demonstrated pumps were only configured for and could only pump gases. There is a need for a micro fluidic pump that can pump liquid from a reservoir inside capillaries using thermal transpiration.
One of the major challenges encountered by the previous Knudsen pump designs is to thermally isolate the hot and cold chambers from each other. The hot chamber is actively heated while the cold chamber temperature is passively cooled. To maintain the cold chamber at room temperature a relatively long channel is used, but this adversely impacts the pump's flow rate. Furthermore, to optimize the pump efficiency, it is desired to thermally insulate the hot chamber to minimize thermal losses, and simultaneously cool the cold chamber. Previous pumps have passively cooled the cold chamber by using large areas to minimize the thermal resistance and by using heat sinks. The need for very different geometries for the hot and cold sides makes it difficult to make an efficient Knudsen pump that is also bi-directional. There is a need for a Knudsen pump, particularly a micro pump, with an actively cooled cold chamber which eliminates the need for a heat sink. Additionally, both hot and cold chambers can be thermally insulated thereby improving efficiency. It is then possible to create an efficient bi-directional Knudsen pump. A further advantage is that the channel length can be minimized, reducing the channel flow conductance and increasing the gas flow rate.
Thermoelectric materials exhibit the Peltier effect. When a voltage is applied across the two ends of the thermoelectric material or in other words current flows through the thermoelectric material a temperature difference is obtained across the same. The charge carriers (electrons) start moving in the opposite direction of the current, transferring the heat with them from one side of the material to the other side and in the process creating a temperature gradient across the material. For example, a thermoelectric or Peltier module is a solid state active heat transfer device which transfers heat from one side of the device to the other when voltage is applied across the device. The direction of heat transfer is controlled by the polarity of the current; therefore, reversing polarity will change the direction of heat transfer. A thermoelectric heat transfer module or device is comprised of one or more thermoelectric materials.
There is a need for a thermally driven micropump such as the Knudsen pump of the present invention. The pump of the present invention has no moving parts, high reliability and uses a low operating voltage. Additionally, the absence of moving parts allows a simple fabrication process. Once the operation has reached equilibrium, the Knudsen pump generates a continuous flow.
It is an objective of the present invention to create a pump with improved flow rates and efficiency as compared to prior art Knudsen pumps. It is another objective of the present invention to create an embodiment of the pump that is a thermoelectric Knudsen pump which utilizes a thermoelectric material to both actively heat the hot chamber and actively cool the cold chamber. It is another objective of the present invention to create an embodiment that is a bi-directional pump whereby the pump direction can be switched by changing the voltage polarity across the thermoelectric material. Another objective of the present invention is to create an embodiment that is a symmetrical pump with no moving parts. Yet another objective of the present invention is to create an embodiment that is a uni-directional pump integrated in a lab-on-chip device with no moving parts that operates using thermal transpiration.